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You searched for: creator:"E. J. Mele"
[texts]Band Symmetries and Singularities in Twisted Multilayer Graphene - E. J. Mele
The electronic spectra of rotationally faulted graphene bilayers are calculated using a continuum formulation for small fault angles that identifies two distinct electronic states of the coupled system. The low energy spectra of one state features a Fermi velocity reduction which ultimately leads to pairwise annihilation and regeneration of its low energy Dirac nodes. The physics in the complementary state is controlled by pseudospin selection rules that prevent a Fermi velocity renormalization ...
Downloads: 3
[texts]Commensuration and Interlayer Coherence in Twisted Bilayer Graphene - E. J. Mele
The low energy electronic spectra of rotationally faulted graphene bilayers are studied using a long wavelength theory applicable to general commensurate fault angles. Lattice commensuration requires low energy electronic coherence across a fault and preempts massless Dirac behavior near the neutrality point. Sublattice exchange symmetry distinguishes two families of commensurate faults that have distinct low energy spectra which can be interpreted as energy-renormalized forms of the spectra for...
Downloads: 4
[texts]Interlayer coupling in rotationally faulted multilayer graphenes - E. J. Mele
This article reviews progress in the theoretical modelling of the electronic structure of rotationally faulted multilayer graphenes. In these systems the crystallographic axes of neighboring layers are misaligned so that the layer stacking does not occur in the Bernal structure observed in three dimensional graphite and frequently found in exfoliated bilayer graphene. Notably, rotationally faulted graphenes are commonly found in other forms of multilayer graphene including epitaxial graphenes th...
Downloads: 10
[texts]Casimir Interactions Between Scatterers in Metallic Carbon Nanotubes - Dina Zhabinskaya
We study interactions between localized scatterers on metallic carbon nanotubes by a mapping onto a one-dimensional Casimir problem. Backscattering of electrons between localized scattering potentials mediates long range forces between them. We model spatially localized scatterers by local and non-local potentials and treat simultaneously the effects of intravalley and intervalley backscattering. We find that the long range forces between scatterers exhibit the universal power law decay of the C...
Downloads: 4
[texts]Nodal Surfaces in Photoemission from Twisted Bilayer Graphene - Anshuman Pal
Selection rules and interference effects in angle resolved photoemission spectra from twisted graphene bilayers are studied within a long wavelength theory for the electronic structure. Using a generic model for the interlayer coupling, we identify features in the calculated ARPES momentum distributions that are controlled by the singularities and topological character of its long wavelength spectrum...
Downloads: 8
[texts]Landau Quantization in Twisted Bilayer Graphenes: the Dirac Comb - M. Kindermann
We study the Landau quantization of the electronic spectrum for graphene bilayers that are rotationally faulted to produce periodic superlattices. Commensurate twisted bilayers exist in two families distinguished by their sublattice exchange parity. We show that these two families exhibit distinct Landau quantized spectra distinguished both by the interlayer coupling of their zero modes and by an amplitude modulation of their spectra at energies above their low energy interlayer coherence scales...
Downloads: 3
[texts]Nanotube Piezoelectricity - Na Sai
We combine ab initio, tight-binding methods and analytical theory to study piezoelectric effect of boron nitride nanotubes. We find that piezoelectricity of a heteropolar nanotube depends on its chirality and diameter and can be understood starting from the piezoelectric response of an isolated planar sheet, along with a structure specific mapping from the sheet onto the tube surface. We demonstrate that coupling between the uniaxial and shear deformation are only allowed in the nanotubes with l...
Downloads: 12
[texts]Electric Polarization of Heteropolar Nanotubes as a Geometric Phase - E. J. Mele
The three-fold symmetry of planar boron nitride, the III-V analog to graphene, prohibits an electric polarization in its ground state, but this symmetry is broken when the sheet is wrapped to form a BN nanotube. We show that this leads to an electric polarization along the nanotube axis which is controlled by the quantum mechanical boundary conditions on its electronic states around the tube circumference...
Downloads: 10
[texts]Broken Symmetries in Scanning Tunneling Images of Carbon Nanotubes - C. L. Kane
Scanning tunneling images of carbon nanotubes frequently show electron distributions which break the local sixfold symmetry of the graphene sheet. We present a theory of these images which relates these anisotropies to the off diagonal correlations in the single particle density matrix, and allows one to extract these correlations from the observed images. The theory is applied to images of the low energy states reflected at the end of a tube or by point defects, and to states propagating on def...
Downloads: 15
[texts]Anisotropic Pairing on a Three-Band Fermi Surface for $A_3{\rm C}_{60}$ - E. J. Mele
We study a model for anisotropic singlet pairing in $A_3{\rm C}_{60}$, using a realistic model for the Fermi surface in a hypothesized orientationally-ordered doped crystal. Anisotropic solutions are studied by combining numerical solutions to the gap equation in the low-temperature phase with a Landau expansion near the mean field critical temperature. We focus on a class of three-dimensional nodeless $d$-wave solutions to the model, which exhibit a fully developed gap everywhere on the Fermi s...
Downloads: 12
[texts]Tight-Binding Parametrization of First-Principles Electronic Dispersion in Orientationally Disordered A3c60 - S. C. Erwin
We derive numerical tight-binding hopping parameters to describe conduction-band dispersion in arbitrary orientational phases of $A_3$C$_{60}$. The parameters are obtained by direct Fourier inversion of the spectra from self-consistent electronic-structure calculations for K$_3$C$_{60}$ using the local-density approximation, including the effects of orientational dependence. Using the new parameters, we revisit several earlier investigations of the orientational ordering in $A_3$C$_{60}$; some o...
Downloads: 7
[texts]Electron Interactions and Scaling Relations for Optical Excitations in Carbon Nanotubes - C. L. Kane
Recent fluorescence spectroscopy experiments on single wall carbon nanotubes reveal substantial deviations of observed absorption and emission energies from predictions of noninteracting models of the electronic structure. Nonetheless, the data for nearly armchair nanotubes obey a nonlinear scaling relation as a function the tube radius $R$. We show that these effects can be understood in a theory of large radius tubes, derived from the theory of two dimensional graphene where the coulomb intera...
Downloads: 7
[texts]The Ratio Problem in Single Carbon Nanotube Fluorescence Spectroscopy - C. L. Kane
The electronic bandgaps measured in fluorescence spectroscopy on individual single wall carbon nanotubes isolated within micelles show signficant deviations from the predictions of one electron band theory. We resolve this problem by developing a theory of the electron hole interaction in the photoexcited states. The one dimensional character and tubular structure introduce a novel relaxation pathway for carriers photoexcited above the fundamental band edge...
Downloads: 5
[texts]Low Energy Coherent Transport in Metallic Carbon Nanotube Junctions - A. A. Maarouf
We study the low-energy electronic properties of a junction made of two crossed metallic carbon nanotubes of general chiralities. We derive a tight binding tunneling matrix element that couples low-energy states on the two tubes, which allows us to calculate the contact conductance of the junction. We find that the intrinsic asymmetries of the junction cause the forward and backward hopping probabilities from one tube to another to be different...
Downloads: 7
[texts]Luttinger Liquid Instability in the One Dimensional t-J Model - C. Stephen Hellberg
We study the t-J model in one dimension by numerically projecting the true ground state from a Luttinger liquid trial wave function. We find the model exhibits Luttinger liquid behavior for most of the phase diagram in which interaction strength and density are varied. However at small densities and high interaction strengths a new phase with a gap to spin excitations and enhanced superconducting correlations is found...
Downloads: 6
[texts]Size, Shape and Low Energy Electronic Structure of Carbon Nanotubes - C. L. Kane
A theory of the long wavelength low energy electronic structure of graphite-derived nanotubules is presented. The propagating $\pi$ electrons are described by wrapping a massless two dimensional Dirac Hamiltonian onto a curved surface. The effects of the tubule size, shape and symmetry are included through an effective vector potential which we derive for this model. The rich gap structure for all straight single wall cylindrical tubes is obtained analytically in this theory, and the effects of ...
Downloads: 4
[texts]$Z_2$ Topological Order and the Quantum Spin Hall Effect - C. L. Kane
The quantum spin Hall (QSH) phase is a time reversal invariant electronic state with a bulk electronic band gap that supports the transport of charge and spin in gapless edge states. We show that this phase is associated with a novel $Z_2$ topological invariant, which distinguishes it from an ordinary insulator. The $Z_2$ classification, which is defined for time reversal invariant Hamiltonians, is analogous to the Chern number classification of the quantum Hall effect...
Downloads: 7
[texts]Quantum Spin Hall Effect in Graphene - C. L. Kane
We study the effects of spin orbit interactions on the low energy electronic structure of a single plane of graphene. We find that in an experimentally accessible low temperature regime the symmetry allowed spin orbit potential converts graphene from an ideal two dimensional semimetallic state to a quantum spin Hall insulator. This novel electronic state of matter is gapped in the bulk and supports the quantized transport of spin and charge in gapless edge states that propagate at the sample bou...
Downloads: 5
[texts]Carbon Nanotubes in Helically Modulated Potentials - P. J. Michalski
We calculate effects of an applied helically symmetric potential on the low energy electronic spectrum of a carbon nanotube in the continuum approximation. The spectrum depends on the strength of this potential and on a dimensionless geometrical parameter, P, which is the ratio of the circumference of the nanotube to the pitch of the helix. We find that the minimum band gap of a semiconducting nanotube is reduced by an arbitrarily weak helical potential, and for a given field strength there is a...
Downloads: 11
[texts]Electron Propagation in Orientationally Disordered Fullerides - E. J. Mele
We study the electronic spectrum for doped electronic states in the orientationally disordered M3C60 fullerides. Momentum-resolved Green's functions are calculated within a cluster-Bethe-lattice model, and compared with results from calculations on periodically repeated supercells containing quenched orientational disorder. Despite the relatively strong scattering from orientational fluctuations, the electronic states near the Fermi energy are well described by propagating states characterized b...
Downloads: 6
[texts]Nonradiative Recombination of Excitons in Carbon Nanotubes Mediated by Free Charge Carriers - J. M. Kinder
Free electrons or holes can mediate the nonradiative recombination of excitons in carbon nanotubes. Kinematic constraints arising from the quasi one-dimensional nature of excitons and charge carriers lead to a thermal activation barrier for the process. However, a model calculation suggests that the rate for recombination mediated by a free electron is the same order of magnitude as that of two-exciton recombination...
Downloads: 3
[texts]Continuum theory for the piezoelectric response of chiral nanotubes under uniaxial and torsional stresses - P. J. Michalski
We develop and solve a continuum theory for the piezoelectric response of nanotubes under applied uniaxial and torsional stresses. We find that the piezoelectric response is controlled by the chiral angle, the aspect ratio, and two dimensionless parameters specifying the ratio of the strengths of the electrostatic and elastic energies. The model is solved in two limiting cases and the solutions are discussed...
Downloads: 4
[texts]Photogalvanic Effects in Heteropolar Nanotubes - P. Kral
We show that an electrical shift current is generated when electrons are photoexcited from the valence to conduction bands on a BN nanotube. This photocurrent follows the light pulse envelope and its symmetry is controlled by the atomic structure of the nanotube. We find that the shift current has an intrinsic quantum mechanical signature in which the chiral index of the tube determines the direction of the current along the tube axis...
Downloads: 13
[texts]Formation of Subgap States in Carbon Nanotubes Due to a Local Transverse Electric Field - Jesse M. Kinder
We introduce two simple models to study the effect of a spatially localized transverse electric field on the low-energy electronic structure of semiconducting carbon nanotubes. Starting from the Dirac Hamiltonian for the low energy states of a carbon nanotube, we use scattering theory to show that an arbitrarily weak field leads to the formation of localized electronic states inside the free nanotube band gap...
Downloads: 9
[texts]Coherent Control of Photocurrents in Graphene and Carbon Nanotubes - E. J. Mele
Coherent one photon ($2 \omega$) and two photon ($ \omega$) electronic excitations are studied for graphene sheets and for carbon nanotubes using a long wavelength theory for the low energy electronic states. For graphene sheets we find that coherent superposition of these excitations produces a polar asymmetry in the momentum space distribution of the excited carriers with an angular dependence which depends on the relative polarization and phases of the incident fields...
Downloads: 8
[texts]Effects of Impurity Vertex Correction on NMR Coherence Peak in S-Wave Superconductors - Han-Yong Choi
We study the effects of non-magnetic impurity vertex correction on nuclear spin-lattice relaxation rate $1/{T_1} $ of conventional s-wave superconductors within the Eliashberg formalism. We obtain, with a self-consistent $t-$matrix treatment of impurity scatterings, the expressions for impurity vertex function and nuclear spin-lattice relaxation rate. The $1/T_1$ is evaluated with a simple approximation on angular average, and found to agree in clean limit with the previous result that $1/(T_1 T...
Downloads: 12
[texts]Casimir Effect for Massless Fermions in One Dimension: A Force Operator Approach - Dina Zhabinskaya
We calculate the Casimir interaction between two short range scatterers embedded in a background of one dimensional massless Dirac fermions using a force operator approach. We obtain the force between two finite width square barriers, and take the limit of zero width and infinite potential strength to study the Casimir force mediated by the fermions. For the case of identical scatterers we recover the conventional attractive one dimensional Casimir force...
Downloads: 12
[texts]Theoretical examination of stress fields in Pb(Zr_0.5Ti_0.5)O_3 - Nicholas J. Ramer
In this paper, we develop a rigorous formulation of the local stress field. This approach can be used in conjuction with any first-principles method to study stress fields in complex bonded systems. In particular we investigate the induced stress fields resulting from homogeneous deformations for tetragonal PbTiO_3 and rhombohedral PbZrO_3. As an extension of these findings we also compute the induced stress fields resulting from homogeneous deformations of the (100) and (111) orderings of Pb(Zr...
Downloads: 4
[texts]Surface States of Topological Insulators - Fan Zhang
We develop an effective bulk model with a topological boundary condition to study the surface states of topological insulators. We find that the Dirac point energy, the band curvature and the spin texture of surface states are crystal face-dependent. For a given face on a sphere, the Dirac point energy is determined by the bulk physics that breaks p-h symmetry in the surface normal direction and is tunable by surface potentials that preserve T symmetry...
Downloads: 3
[texts]Three-dimensional electronic instabilities in polymerized solid A1C60 - S. C. Erwin
The low-temperature structure of A1C60 (A=K, Rb) is an ordered array of polymerized C60 chains, with magnetic properties that suggest a non-metallic ground state. We study the paramagnetic state of this phase using first-principles electronic-structure methods, and examine the magnetic fluctuations around this state using a model Hamiltonian. The electronic and magnetic properties of even this polymerized phase remain strongly three dimensional, and the magnetic fluctuations favor an unusual thr...
Downloads: 19
[texts]Commensurate and modulated magnetic phases in orthorhombic A1C60 - E. J. Mele
Competing magnetically ordered structures in polymerized orthorhombic A1C60 are studied. A mean-field theory for the equilibrium phases is developed using an Ising model and a classical Heisenberg model to describe the competition between inter- and intra-chain magnetic order in the solid. In the Ising model, the limiting commensurate one-dimensional and three-dimensional phases are separated by a commensurate three-sublattice state and by two sectors containing higher-order commensurate phases...
Downloads: 18
[texts]Continuum Theory for Piezoelectricity in Nanotubes and Nanowires - P. J. Michalski
We develop and solve a continuum theory for the piezoelectric response of one dimensional nanotubes and nanowires, and apply the theory to study electromechanical effects in BN nanotubes. We find that the polarization of a nanotube depends on its aspect ratio, and a dimensionless constant specifying the ratio of the strengths of the elastic and electrostatic interactions. The solutions of the model as these two parameters are varied are discussed...
Downloads: 9
[texts]Valley Chern Numbers and Boundary Modes in Gapped Bilayer Graphene - Fan Zhang
Electronic states at domain walls in bilayer graphene are studied by analyzing their four and two band continuum models, by performing numerical calculations on the lattice, and by using quantum geometric arguments. The continuum theories explain the distinct electronic properties of boundary modes localized near domain walls formed by interlayer electric field reversal, by interlayer stacking reversal, and by simultaneous reversal of both quantities...
Downloads: 9
[texts]Time Reversal Invariant Topological Superconductivity and Majorana Kramers Pairs - Fan Zhang
We propose a feasible route to engineer one and two dimensional time reversal invariant (TRI) topological superconductors (SC) via proximity effects between nodeless extended s wave iron-based SC and semiconductors with large Rashba spin-orbit interactions. At the boundary of a TRI topological SC, there emerges a Kramers pair of Majorana edge (bound) states. For a Josephson pi-junction we predict a Majorana quartet that is protected by mirror symmetry and leads to a mirror fractional Josephson e...
Downloads: 11
[texts]Topological Insulators in Three Dimensions - Liang Fu
We study three dimensional generalizations of the quantum spin Hall (QSH) effect. Unlike two dimensions, where the QSH effect is distinguished by a single $Z_2$ topological invariant, in three dimensions there are 4 invariants distinguishing 16 "topological insulator" phases. There are two general classes: weak (WTI) and strong (STI) topological insulators. The WTI states are equivalent to layered 2D QSH states, but are fragile because disorder continuously connects them to band insulators...
Downloads: 11
[texts]Topological Mirror Superconductivity - Fan Zhang
We demonstrate the existence of topological superconductors (SC) protected by mirror and time reversal (TR) symmetries. D-dimensional (D=1,2,3) crystalline SCs are characterized by 2^(D-1) independent integer topological invariants, which take the form of mirror Berry phases. These invariants determine the distribution of Majorana modes on a mirror symmetric boundary. The parity of total mirror Berry phase is the Z_2 index of a class DIII SC, implying that a DIII topological SC with a mirror lin...
Downloads: 9
[texts]Common path interference in Zener tunneling is a universal phenomenon - Sonika Johri
We show that the probability of electric field induced interband tunneling in solid state systems is generically a non-monotonic (oscillatory) function of the applied field. This unexpected behavior can be understood as arising due to a common path interference between two distinct tunneling solutions. The phenomenon is insensitive to magnetic field, and arises whenever the low energy dispersion relation contains higher order terms in addition to the usual $p^2$ term...
Downloads: 15
[texts]Three-dimensional electronic instabilities in polymerized solid A1C60 - S. C. Erwin
The low-temperature structure of A1C60 (A=K, Rb) is an ordered array of polymerized C60 chains, with magnetic properties that suggest a non-metallic ground state. We study the paramagnetic state of this phase using first-principles electronic-structure methods, and examine the magnetic fluctuations around this state using a model Hamiltonian. The electronic and magnetic properties of even this polymerized phase remain strongly three dimensional, and the magnetic fluctuations favor an unusual thr...
Downloads: 7
[texts]Midinfrared Conductivity in Orientationally Disordered Doped Fullerides - M. S. Deshpande
The coupling between the intramolecular vibrational modes and the doped conduction electrons in $M_3C_{60}$ is studied by a calculation of the electronic contributions to the phonon self energies. The calculations are carried out for an orientationally ordered reference solid with symmetry $Fm \bar{3} m$ and for a model with quenched orientational disorder on the fullerene sites. In both cases, the dispersion and symmetry of the renormalized modes is governed by the electronic contributions...
Downloads: 5
[texts]Surface Potentials and Layer Charge Distributions in Few-Layer Graphene Films - Sujit S. Datta
Graphene-derived nanomaterials are emerging as ideal candidates for postsilicon electronics. Elucidating the electronic interaction between an insulating substrate and few-layer graphene (FLG) films is crucial for device applications. Here, we report electrostatic force microscopy (EFM) measurements revealing that the FLG surface potential increases with film thickness, approaching a "bulk" value for samples with five or more graphene layers...
Downloads: 39
[texts]Continuum Elastic Theory of Adsorbate Vibrational Relaxation - Steven P. Lewis
An analytical theory is presented for the damping of low-frequency adsorbate vibrations via resonant coupling to the substrate phonons. The system is treated classically, with the substrate modeled as a semi-infinite elastic continuum and the adsorbate overlayer modeled as an array of point masses connected to the surface by harmonic springs. The theory provides a simple expression for the relaxation rate in terms of fundamental parameters of the system: $\gamma = m\bar{\omega}_0^2/A_c \rho c_T$...
Downloads: 3
[texts]Effective-Medium Theory for the Normal State in Orientationally Disordered Fullerides - M. S. Deshpande
An effective-medium theory for studying the electronic structure of the orientationally disordered A3C60 fullerides is developed and applied to study various normal-state properties. The theory is based on a cluster-Bethe-lattice method in which the disordered medium is modelled by a three-band Bethe lattice, into which we embed a molecular cluster whose scattering properties are treated exactly. Various single-particle properties and the frequency-dependent conductivity are calculated in this m...
Downloads: 5
[texts]Chirality Dependence of the $K$-Momentum Dark Excitons in Carbon Nanotubes - P. M. Vora
Using a collection of twelve semiconducting carbon nanotube samples, each highly enriched in a single chirality, we study the chirality dependence of the $K$-momentum dark singlet exciton using phonon sideband optical spectroscopy. Measurements of bright absorptive and emissive sidebands of this finite momentum exciton identify its energy as 20 - 38 meV above the bright singlet exciton, a separation that exhibits systematic dependencies on tube diameter, $2n+m$ family, and semiconducting type...
Downloads: 8
[texts]Nanoparticle Shape Selection By Repulsive Interactions: Metal Islands on Few Layer Graphenes - L. A. Somers
Metal atoms adsorbed on few layer graphenes condense to form nanometer-size droplets whose growth is size limited by a competition between the surface tension and repulsive electrostatic interactions from charge transfer between the metal droplet and the graphene. For situations where the work function mismatch is large and the droplet surface tension is small, a growing droplet can be unstable to a family of shape instabilities...
Downloads: 27
[texts]Reciprocity Theorems for Ab Initio Force Calculations - C. Wei
We present a method for calculating ab initio interatomic forces which scales quadratically with the size of the system and provides a physically transparent representation of the force in terms of the spatial variation of the electronic charge density. The method is based on a reciprocity theorem for evaluating an effective potential acting on a charged ion in the core of each atom. We illustrate the method with calculations for diatomic molecules.
Downloads: 12
[texts]Structure and Vibrations of the Vicinal Copper (211) Surface - C. Y. Wei
We report a first principles theoretical study of the surface relaxation and lattice dynamics of the Cu(211) surface using the plane wave pseudopotential method. We find large atomic relaxations for the first several atomic layers near the step edges on this surface, and a substantial step-induced renormalization of the surface harmonic force constants. We use the results to study the harmonic fluctuations around the equilibrium structure and find three new step-derived features in the zone cent...
Downloads: 24
[texts]Size-selective nanoparticle growth on few-layer graphene films - Zhengtang Luo
We observe that gold atoms deposited by physical vapor deposition onto few layer graphenes condense upon annealing to form nanoparticles with an average diameter that is determined by the graphene film thickness. The data are well described by a theoretical model in which the electrostatic interactions arising from charge transfer between the graphene and the gold particle limit the size of the growing nanoparticles...
Downloads: 4
[texts]Dirac semimetal in three dimensions - S. M. Young
In a Dirac semimetal, the conduction and valence bands contact only at discrete (Dirac) points in the Brillouin zone (BZ) and disperse linearly in all directions around these critical points. Including spin, the low energy effective theory around each critical point is a four band Dirac Hamiltonian. In two dimensions (2D), this situation is realized in graphene without spin-orbit coupling. 3D Dirac points are predicted to exist at the phase transition between a topological and a normal insulator...
Downloads: 12
[texts]Temperature Dependent Resistivity of Single Wall Carbon Nanotubes - C. L. Kane
Nonchiral single wall carbon nanotubes with an "armchair" wrapping are theoretically predicted to be conducting, and high purity samples consisting predominantly of these tubes exhibit metallic behavior with an intrinsic resistivity which increases approximately linearly with temperature over a wide temperature range. Here we study the coupling of the conduction electrons to long wavelength torsional shape fluctuations, or twistons...
Downloads: 9
[texts]Probing Spin-Charge Relation by Magnetoconductance in One-Dimensional Polymer Nanofibers - A. Choi
Polymer nanofibers are one-dimensional organic hydrocarbon systems containing conducting polymers where the non-linear local excitations such as solitons, polarons and bipolarons formed by the electron-phonon interaction were predicted. Magnetoconductance (MC) can simultaneously probe both the spin and charge of these mobile species and identify the effects of electron-electron interactions on these nonlinear excitations...
Downloads: 8
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